﻿Report 
  of 
  the 
  State 
  Geologist, 
  

  

  903 
  

  

  46 
  

  

  This 
  genus 
  differs 
  from 
  the 
  typical 
  Fenestella 
  as 
  follows 
  : 
  The 
  

   carina 
  is 
  always 
  very 
  prominent, 
  being 
  frequently 
  equal 
  in 
  height 
  

   to 
  twice 
  the 
  thickness 
  of 
  the 
  branches 
  or 
  even 
  more. 
  For 
  a 
  

   short 
  distance 
  above 
  the 
  branches, 
  generally 
  for 
  about 
  one-third 
  

   or 
  one 
  fourth 
  of 
  its 
  height, 
  the 
  carina 
  is 
  very 
  thin 
  ; 
  it 
  then 
  becomes 
  

   thickened 
  and'continues 
  of 
  essentially 
  

   the 
  same 
  width 
  to 
  the 
  summit. 
  The 
  

   thickened 
  portion 
  is 
  usually 
  a 
  little 
  

   more 
  than 
  twice 
  the 
  width 
  of 
  the 
  

   thinner 
  portion, 
  rarely 
  more. 
  The 
  

   carinae 
  are 
  connected 
  by 
  thin, 
  usually 
  

   oblique 
  plates, 
  called 
  sealae, 
  which 
  

  

  extend 
  the 
  whole 
  height 
  of 
  the 
  thickened 
  portion 
  of 
  the 
  carinae. 
  

   In 
  some 
  species 
  the 
  entire 
  plate 
  is 
  oblique 
  to 
  the 
  axis 
  of 
  the 
  

   branches 
  in 
  one 
  direction 
  (fig. 
  [46); 
  in 
  others 
  the 
  plate 
  is 
  abruptly 
  

   bent 
  at 
  about 
  half 
  of 
  its 
  height, 
  so 
  that 
  the 
  upper 
  half 
  of 
  the 
  

   sealae 
  is 
  oblique 
  to 
  the 
  branch 
  in 
  an 
  opposite 
  direction 
  of 
  the 
  

   lower 
  (figs. 
  47, 
  48.) 
  

  

  47 
  

  

  48 
  

  

  In 
  these 
  cases 
  the 
  lower 
  portion 
  of 
  the 
  sealae 
  is 
  very 
  thin, 
  the 
  

   upper 
  portion 
  being 
  two 
  or 
  three 
  times 
  the 
  thickness 
  of 
  the 
  

   lower. 
  In 
  those 
  cases 
  where 
  the 
  sealae 
  are 
  not 
  bent, 
  the 
  lower 
  

   portion 
  or 
  base 
  is 
  the 
  thickest, 
  gradually 
  growing 
  less 
  to 
  the 
  

   summit, 
  which 
  is 
  very 
  thin. 
  

  

  The 
  sealae 
  frequently 
  correspond 
  in 
  number 
  to 
  the 
  cell 
  aper- 
  

   tures, 
  but 
  this 
  may 
  be 
  only 
  accident, 
  as 
  in 
  other 
  cases 
  the 
  cell 
  

   apertures 
  are 
  from 
  two 
  to 
  four 
  times 
  as 
  numerous 
  as 
  the 
  sealae. 
  

  

  In 
  a 
  superficial 
  examination 
  it 
  might 
  seem 
  that 
  the 
  generic 
  

   characters 
  of 
  Unitrypa 
  and 
  Isotrypa 
  were 
  the 
  same, 
  but 
  in 
  reality 
  

   they 
  are 
  very 
  different. 
  

  

  In 
  Isotrypa 
  the 
  summits 
  only 
  of 
  the 
  carinae 
  are 
  connected 
  

   by 
  bars, 
  which 
  do 
  not 
  extend 
  upon 
  the 
  sides 
  of 
  the 
  carinae. 
  

  

  